Dental restorative materials which are cured by exposure to light must possess certain physical and chemical characteristics to be suitable for use by a dentist in filling, restoring or repairing teeth. Dental restorative or composite photocurable materials which are used for filling and repairing teeth should be distinguished from dental compositions used as a bonding agent or to form a glaze or thin coating. In the latter case, the compositions are substantially "unfilled" and of relatively low viscosity, whereas dental restorative materials for filling and restoring teeth are viscous materials containing a substantial concentration of inorganic particulate filler to provide a high viscosity. The photocurable resinous component(s) in the restorative material is homogeneously distributed throughout the composition and must receive sufficient light energy to cause the material to cure uniformly from the top to the bottom of the restoration. There are a substantial number of commercially available curing lamps in the marketplace, all having different light-generating characteristics within the light energy spectrum corresponding to the material to be cured. Moreover, commercially available dental restorative materials come in various shades with different fillers. They have different aging and shelf-life characteristics and tend to degrade in performance or deteriorate at varying temperature levels above ambient.
Accordingly, there is a concern by the dental practitioner as to whether a given restorative material, which the dentist currently has in stock, will, upon the application of light energy, harden throughout the restoration in a reasonable time frame. Currently there is no convenient way for a dentist to be assured that the restorative material available in the dental office has not aged or deteriorated, such that it will not cure properly under the curing lamp or, alternatively, if the curing lamp is capable of curing the material in the time frame specified by the material manufacturer. This is particularly the case for deep posterior restorations which require more light energy and time to effect a cure at the bottom of the restoration. Otherwise, the bottom of the restoration will remain uncured or only partially cured. An uncured or only partially cured restoration does not have the requisite hardness to serve as a dental restoration. This problem comes about because the curing light cures the material from the top down, i.e., the outside proximal surface hardens first, with the lower distal surface furthermost removed from the source of the curing energy curing last. Extensive research has clearly shown that the degree of hardness of a dental restorative material is directly related to its degree of cure. If the restorative material in the area adjacent the pulp chamber base of a tooth cavity is not cured properly, the overall restoration will be physically weak and micro-leakage may occur, as well as patient sensitivity due to the presence of uncured materials which also serves as an irritant to the dental pulp.